1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging apparatus for obtaining nuclear magnetic resonance images of an object to be examined.
2. Description of the Background Art
As is well known, in a nuclear magnetic resonance imaging apparatus, a nuclear magnetic resonance image is obtained by placing an object to be examined in a static magnetic field; applying a high frequency magnetic field (RF pulse) in a direction perpendicular to that of the static magnetic field in order to induce a nuclear magnetic resonance phenomenon in the object to be examined; superposing gradient magnetic fields G.sub.X, G.sub.Y, and G.sub.Z in X, Y, and Z directions, respectively, onto the static magnetic field, for the sake of tomographic imaging; collecting nuclear magnetic resonance signals due to the induced nuclear magnetic resonance phenomenon from the object to be examined; and image processing the collected nuclear magnetic resonance signals.
In such a nuclear magnetic resonance imaging apparatus, the gradient magnetic fields G.sub.X, G.sub.Y, and G.sub.Z are produced by using X- , Y-, and Z-gradient magnetic field coils provided in correspondence with X, Y, and Z axes, respectively, each of which is equipped with an independent power source of the same power capacity.
In terms of functions, the gradient magnetic fields can be considered as comprising three orthogonal fields of a slicing gradient magnetic field G.sub.S for determining a slicing plane of tomographic imaging, a phase encoding gradient magnetic field G.sub.E for providing coordinate information on the slicing plane, and a reading gradient magnetic field G.sub.R for tomographic extraction of the nuclear magnetic resonance signals.
These gradient magnetic fields are obtained as a field given by superposition of three orthogonal gradient magnetic fields G.sub.X, G.sub.Y, and G.sub.Z in X, Y, and Z directions, respectively. For example, a total gradient magnetic field G.sub.0 shown in FIG. 1 can be obtained from three orthogonal gradient magnetic fields G.sub.X, G.sub.Y, and G.sub.Z, and this total gradient magnetic field G.sub.0 can be taken as composed from three components corresponding to the slicing gradient magnetic field G.sub.S, the phase encoding gradient magnetic field G.sub.E, and the reading gradient magnetic field G.sub.R.
Now, in a conventional nuclear magnetic resonance imaging apparatus, each of X-, Y-, and Z-gradient magnetic field coils is equipped with an independent power source of the same power capacity, so that, by assuming that a maximum power of each power source to be 1, a total power capacity of these power sources is equal to 3. However, in order to be able to take an image at an arbitrary cross section, a maximum total power required from these power sources is at most 1/.sqroot.3.times.3=.sqroot.3.apprxeq.1.73 occurring in a case of a total gradient magnetic field obliquely inclined by 45.degree. from all of X, Y, and Z axes.
Thus, in a conventional nuclear magnetic resonance imaging apparatus, over 40% of the total power capacity of the power sources for the gradient magnetic field coils has always been wasted as unproductive power capacity.
This situation is particularly problematic in using a modern imaging technique, such as an echo planer method, in which the required power for the reading gradient magnetic field is much larger than the required powers for the slicing gradient magnetic field and the phase encoding gradient magnetic field. In such a case, the conventional provision of providing three independent power sources of the same power capacities produces a waste of a very large amount of power.